PUB - Publikationen an der Universität Bielefeld

A series of π-conjugated systems with two and three 1,3-diethyl-1,3,2-benzodiazaborolyl end-groups was synthesised in 58-91% yields using established 1,3,2-diazaborole methodologies. The bis(diazaborolyl) compounds contain thiophene -2,5-C4H2S- (2a), dithiophene -5,5'-(2,2'-C4H2S)(2)- (2b), phenylene -1,4-C6H4- (2c), biphenylene - 4,4'-(1,1'-(C6H4)(2))- (2d) and dioctylfluorene -2,7-(9,9-(C8H7)(2)C11H6)- (2e) bridges. The three-way linkers in the tris(diazaborolyl) assemblies contain a central phenylene unit - 1,3,5-C6H3- linked to the borolyl end groups via thiophene - 2,5-C4H2S- (3a), directly bonded (3b) or via phenylene - 1,4-C6H4- (3c) units. Molecular structures of 2a, 2b, 2c, 3a, 3b and 3c were determined by X-ray crystallographic studies. These borolylated systems show intense blue/violet luminescence with Stokes shifts of 6200-9500 cm(-1) and quantum yields of 0.33 to 0.98. The absorption maxima (296-351 nm) of these assemblies are reproduced well by TD-DFT computations (B3LYP/6-31G*), and arise from strong, low energy HOMO-LUMO transitions. From molecular orbital computations on optimised geometries of these diazaborolyl systems, the LUMO is located mainly on the thiophene/benzene bridge (66-92%) while the HOMO is largely benzodiazaborolyl in character (53-83%). The S-1 <- S-0 absorption bands are thus assigned to π(diazaborolyl)-π*(thiophene/benzene) transitions. Computations on related bis(boryl) dithiophenes [ with diarylboryl e. g. Ph2B, Mes(2)B, (C6F5)(2)B and FMes(2)B (Mes = 2,4,6-Me3C6H2; FMes = 2,4,6-(CF3)(3)C6H2), dioxaborolyl and other diazaborolyl groups] reveal strong, low energy UV-visible absorption bands arising from π(thiophene)-π*(thiophene) transitions, with increasing boron participation in the LUMO of the diarylboryl and especially the highly fluorinated systems.